Pope Francis prayed for peace in Syria and Ukraine in his traditional Easter Mass in St Peter's Square on Sunday.

In his Easter message known as Urbi et Orbi (to the city and the world), the pontiff prayed for "the defenceless civil population" in Syria and called on the international community to "boldly negotiate the peace long awaited and long overdue".

After three years of civil war, the death toll in Syria has exceeded 150,000 and forced millions to flee their homes.

Pope Francis also asked God to "enlighten and inspire" peace in Ukraine as the diplomatic stand-off between Moscow and Kiev continues unabated.

His prayer comes just hours after a deadly gunfire battle erupted in the eastern Ukraine town of Slavyansk on Sunday morning, killing at least three and prompting fears that the Easter truce could take a violent turn.

The Argentine pope recalled those suffering in Africa from an epidemic of deadly Ebola, urged a halt to "brutal terrorist attacks" in parts of Nigeria and called for "reconciliation and fraternal accord" in Venezuela, which is experiencing the worst anti-government protests in a decade.

Reflecting the priorities of his papacy, the pontiff prayed for the poor and the abandoned, encouraging the faithful to help "those crushed by life's troubles, share with the needy, stand at the side of the sick, elderly and the outcast".

More than 150,000 tourists and pilgrims gathered in St. Peter's Square for Easter Sunday mass, the culmination of Holy Week, which celebrates the Christian belief that Jesus rose from the dead after his crucifixion.